UCLA

This thesis presents a study on the formation mechanism of porous Cu3Sn intermetallic compounds in microbumps for 3D-IC packaging, which are issues of importance to the packaging technology in microelectronic industry. Experimental emphases are on the formation processes of porous Cu3Sn under current stressing and by solid state aging. Theoretical interpretations of the observations are also presented.

Following a background introduction, we first report the results on electromigration tests of SnAg solder bump samples with 15 �m bump height and Cu under-bump-metallization (UBM). The test conditions were 1.45 �104 A/cm2 at 185 oC and 1.20 �104 A/cm2 at 0 oC. A porous Cu3Sn intermetallic compound (IMC) structure was observed to form within the bumps after several hundred hours of current stressing. In direct comparison, annealing alone at 185 oC will take more than 1000 h for porous Cu3Sn to form, and it will not form at 170 oC even after 2000 h. A mechanism is proposed to explain the formation of this porous structure assisted by electromigration. The results show that the SnAg bump with low bump height will become porous-type Cu3Sn when stressing with high current density and high temperature. Polarity effects on porous Cu3Sn formation is also examined and interpreted.

The second part of the experimental study is on the growth competition between the co-existing layer-type and porous-type Cu3Sn in solder microbumps of Cu/SnAg/Cu. The thickness of the SnAg solder is about 14 μm and the Cu column on both sides is 20 μm. Upon wetting-reflow, the solder is reacted completely to form Cu-Sn intermetallic compounds in a multi-layered structure of Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu. Upon further annealing at 220 �C and 260 �C, we obtain Cu/Cu3Sn/porous Cu3Sn/Cu3Sn/Cu, in which both types of Cu3Sn co-exist and form an interface. In the layer-type growth, we assume Cu to be the dominant diffusing species, coming from the Cu column. The Cu reacts with Cu6Sn5 to grow the Cu3Sn layer. In the porous-type growth, we assume Sn to be the dominant diffusing species, coming from the depletion of Sn in Cu6Sn5. The depleted Cu6Sn5 transforms to the porous-type Cu3Sn. At the same time, the Sn diffuses to the side-wall of Cu column to form a coating of Cu3Sn. Experimental observations of 3-dimensional distribution of voids in the porous-type Cu3Sn are performed by synchrotron radiation tomography; the voids are interconnected for the out-diffusion of Sn. The competing growth between the layer-type and the porous-type Cu3Sn is analyzed.

Lastly, for a theoretical understanding, we discuss the mechanism of this unique porous void formation in microbumps, including the necessary and sufficient conditions of porous Cu3Sn formation and a comparison between incomplete and complete cellular precipitations.